Energetics Flashcards
What energy change is breaking bonds associated with?
Energy is taken in to break bonds ==> endothermic reaction
What energy change is making bonds associated with?
Energy is released to make bonds==> exothermic reaction
What are some uses of thermochemistry?
Measuring and comparing the energy values of fuels
Calculating the energy requirements for industrial processes
Working out the theoretical amount of energy released/taken in a reaction
Predicting if a reaction will take place or not
What is an endothermic reaction?
One with an overall positive enthalpy change => energy in breaking bonds> energy out making bonds
What is an exothermic reaction?
One with an overall negative enthalpy change => energy in breaking bonds< energy out making bonds
If a reversible reaction is endothermic one way, what type of reaction is the other way?
exothermic
Give two examples of exothermic reactions
Combustion of fuels
Neutralization
Give an example of an endothermic reaction.
Thermal decomposition
Define enthalpy change; what symbol is used to represent it?
Energy change of a system at a constant pressure presented by delta H
What are the standard conditions?
100kPa/ 1atm pressure
298 K/ 25 degrees Celsius temperature
What does “ in standard state” mean?
The state an element/ compound exists at standard conditions (100kPa, 298 K)
Draw an enthalpy change diagram for an endothermic reaction, and one for an exothermic reaction.
Look for these on PMT
Define standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions, with reactants and products in their standard states
Give an examples of an equation which represents standard enthalpy of formation
H2 (g) + 1/2 O2 (g) => H2O (Il)
Define standard enthalpy of combustion
The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions, with reactants and products in their standard states.
Give an example of an equation which represents standard enthalpy of combustion.
C (s) + O2 (g) => CO2 (g)
What is the difference between heat and temperature?
Heat is the sum of all particles’ energy, therefore it is affected by the amount of substance; temperature is related to the mean kinetic energy of the particles in a system, so is independent of the number of particles present.
How can you calculate enthalpy change from experimental data?
Use the equation Q =mcT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18 g J^{-1} K^{-1} and T is the change in temperature
Draw a simple calorimeter
Look for this on PMT
How can a calorimeter be made more accurate?
- Try reducing heat loss by adding mineral wool around the beaker
- Add draught screens at the sides
- Or add a lid on top of the beaker if not present
What is a flame calorimeter; how does it differ to a simple calorimeter?
Reduces heat lost to the surrounding to give more accurate results: has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not in air.
How would one measure the enthalpy change for a reaction occurring in (aq)?
- Use an expanded polystyrene cup as a calorimeter (good insulator- which reduces heat loss)
- Heat is generated in the solution; measure this temperature change.
- Take heat capacity of solution to be 4.18 and density of solution = 1 gcm^{-3`}
What can one use to make experimental determination of enthalpy change of reaction more accurate?
Cooling curves
What is Hess’s Law?
states that the enthalpy change for a reaction is the same regardless of the route taken
What is the enthalpy of an element?
The enthalpy of all elements in their standard states (the states in which they exist at 100 kPa and 298 K) is defined as 0
Define bond dissociation enthalpy.
The enthalpy change required to break a covalent bond, with all species in the gaseous state; differs for the same bond type in different molecules
Define mean bond enthalpy.
Average value(across different chemical environments) for the bond dissociation enthalpy of a given bond.
Why may experimental methods for enthalpy determination not be very accurate?
- Heat is lost to the surroundings
- Not in standard conditions
- Reaction may not go to completion
Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?
Bond enthalpies are a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate
